As a result of the original arrangements for administering the system, the registration districts were, and still are, largely identical to the old Poor Law Unions. As these were based on natural catchment areas, normally consisting of a large market town and its rural hinterland, rather than on the already existing administrative divisions of townland, parish and county, registration districts for births, marriages and deaths cut right across these earlier boundaries - a fact that can be very significant for research. For example, Waterford registration district, centred in the town of Waterford, also takes in a large part of rural south Co. Kilkenny; Ballinasloe district contains areas in both south Roscommon and east Galway, Boyle includes parts of Roscommon and Sligo.
A Ireland-wide map of registration districts is on this site, here.
The only comprehensive guide to which towns and townlands are contained in each registration district is to be found in a series of pamphlets produced in the nineteenth century by the Registrar-General's Office for the use of each of the local registrars. These are collected as Townlands in poor law unions : a reprint of poor law union pamphlets of the general registrar's office with an introduction, and six appendices relating to Irish genealogical research, (repr. Ed. Handran, George B. 1997 Higginson, Salem Ma.) copies of which can be found in the National Library (reference: Ir 9141 b 35) or on Eneclann CD-ROM. .This work is particularly useful when a problem arises in identifying a variant version of a townland name given in the original register entry for a marriage, birth or death. By scanning the lists of townlands in the relevant district in which the entry is recorded, it is almost always possible to identify the standard version of the name and, from this, go on to census, parish and land records. A version of the pamphlets is the basis of this site's click-through Poor Law Union map.
To go in the other direction, to find out what registration district a particular town or townland is in, the standard source is the Alphabetical Index to the Towns, Townlands and Parishes of Ireland. Three editions of this were published, based on the census returns for 1851, 1871, and 1901. In the first two, the registration district is recorded as the Poor Law Union; in the 1901 Index it does not appear in the body of the work, but may be found as an appendix. Copies of these can be found on open access in the National Library, the National Archives, The General Register Office itself, or in any library. The 1851 is by far the most widely available, and an expanded version can be found here at www.johngrenham.com/places. The 1901 is on the Irish Genealogical Research Society website www.irishancestors.ie.